Wearable audio device zero-crossing based parasitic oscillation detection

Abstract

A system for detecting parasitic oscillation in a wearable audio device that includes an electro-acoustic transducer that is configured to develop sound for a user, a housing that holds the transducer, at least one of a feedforward microphone that is configured to detect sound outside of the housing and output a feedforward microphone signal and a feedback microphone that is configured to detect sound inside of the housing and output a feedback microphone signal, and an opening in the housing that emits sound pressure from the transducer. The system includes a parasitic oscillation detector that is configured to determine a fundamental frequency of at least one of the feedforward and feedback microphone signals and compare an amplitude of the determined fundamental frequency to a threshold level, to determine parasitic oscillation.

What is claimed is: 
     
       1. A system for detecting parasitic oscillation in a wearable audio device that comprises an electro-acoustic transducer that is configured to develop sound for a user, a housing that holds the transducer, at least one of a feedforward microphone that is configured to detect sound outside of the housing and output a feedforward microphone signal or a feedback microphone that is configured to detect sound inside of the housing and output a feedback microphone signal, and an opening in the housing that emits sound pressure from the transducer, the system comprising:
 a parasitic oscillation detector that is configured to:
 determine a fundamental frequency of at least one of the feedforward and feedback microphone signals; and 
 compare an amplitude of the determined fundamental frequency to a threshold level, to determine parasitic oscillation. 
 
 
     
     
       2. The system of  claim 1  wherein the wearable audio device comprises an earbud that is configured to output sound directly into the user's ear canal. 
     
     
       3. The system of  claim 1  wherein a microphone is used in an active noise reduction (ANR) system. 
     
     
       4. The system of  claim 1  wherein a feedforward microphone is used in a transparency mode where environmental sounds are reproduced by the transducer. 
     
     
       5. The system of  claim 1  wherein the fundamental frequency is determined based on zero crossings of a microphone signal. 
     
     
       6. The system of  claim 5  wherein the fundamental frequency is determined by measuring a number of samples of a running clock between zero crossings. 
     
     
       7. The system of  claim 5  wherein the fundamental frequency is determined based on a monitoring of zero crossings over time. 
     
     
       8. The system of  claim 5  wherein zero crossings are determined based on changes in sign of the microphone signal. 
     
     
       9. The system of  claim 1  wherein the parasitic oscillation detector is further configured to determine whether the fundamental frequency is at least at the threshold level for at least a predetermined amount of time. 
     
     
       10. The system of  claim 1  wherein the parasitic oscillation detector is configured to detect parasitic oscillations in a predetermined frequency range. 
     
     
       11. The system of  claim 10  wherein the frequency range is from about 300 Hz to about 1,000 Hz. 
     
     
       12. The system of  claim 1  further comprising an instability mitigator that is configured to alter a microphone signal in response to a determination of parasitic oscillation. 
     
     
       13. The system of  claim 12  wherein the instability mitigator is configured to mute the microphone. 
     
     
       14. The system of  claim 13  wherein the microphone is muted for a predetermined amount of time. 
     
     
       15. The system of  claim 14  wherein after the predetermined amount of time the microphone is returned to an un-muted state. 
     
     
       16. A system for detecting parasitic oscillation in an earbud that is configured to output sound directly into the user's ear canal, wherein the earbud comprises an electro-acoustic transducer that is configured to develop sound for a user, a housing that holds the transducer, a feedforward microphone that is configured to detect sound outside of the housing and output a feedforward microphone signal that is used in a transparency mode where environmental sounds are reproduced by the transducer, a feedback microphone that is configured to detect sound inside of the housing and output a feedback microphone signal that is used for active noise reduction, and an opening in the housing that emits sound pressure from the transducer that can reach the feedforward microphone, the system comprising:
 a parasitic oscillation detector that is configured to:
 determine a fundamental frequency of a microphone signal based on zero crossings of the microphone signal; 
 compare an amplitude of the fundamental frequency of the microphone signal to a threshold level; and 
 determine whether the fundamental frequency is at least at the threshold level for at least a predetermined amount of time, to determine parasitic oscillation. 
 
 
     
     
       17. The system of  claim 16  wherein the fundamental frequency is determined by measuring a number of samples of a running clock between zero crossings. 
     
     
       18. The system of  claim 16  wherein the fundamental frequency is determined based on a monitoring of zero crossings over time. 
     
     
       19. The system of  claim 16  wherein zero crossings are determined based on changes in sign of the microphone signal. 
     
     
       20. The system of  claim 16  wherein the parasitic oscillation detector is configured to detect parasitic oscillations in a frequency range of from about 300 Hz to about 1,000 Hz.